1. Field of the Invention
The present invention relates to a seat back of vehicle seat or automotive seat. In particular, the invention is directed to a seat back of the type workable to prevent or minimize forward and rearward swinging or rocking of a seat occupant's upper body portion under a normal condition where a rearward low load is repeatedly applied from the seat occupant to the seat back, and also absorb an excessive great rearward load applied from the seat occupant to the seat back in the event of rear-end collision or the like to thereby protect the seat occupant against damage and injury.
2. Description of Prior Art
Among the seat backs of the above-stated kind, there is known a seat back with an active headrest arrangement, which is so designed that, when a rear-end collision occurs, a support element in the seat back receives a back portion of seat occupant being displaced rearwardly thereto under his or her inertia and is then displaced rearwardly to absorb a great impact given to the seat occupant's back portion, while simultaneously a headrest is moved forwardly to receive a head of the seat occupant via an active headrest mechanism. Such seat back is for example disclosed in Japanese Laid-Open Publication No. 2006-341802 (JP 2006-341802 A).
According to the foregoing JP 2006-341802 A, a seat back structure is disclosed, which includes: a seat back frame of substantially rectangular shape having a spaced defined inwardly thereof; a support wire element resiliently supported within such space of the seat back frame in a suspended manner; and a support plate provided anteriorly of the support wire element. A foam padding is disposed anteriorly of and laid on that support plate within the seat back. Basically, with such structure, the back portion of a seat occupant is resiliently supported by the support wire element via the foam padding.
The support wire element is at the upper region thereof connected via a pair of coil springs to a pair of side frame members of the seat back frame, respectively, while being at the lower region thereof connected via a pair of linear spring wires to those two side frame members, respectively. Hence, normally, the upper portion (dorsal portion) of seat occupant is resiliently supported by the two coil springs, whereas the lower portion (lumbar portion) thereof resiliently supported by the two spring wires. Such spring arrangement works effectively in the case of rear-end collision. Upon occurrence of rear-end collision, an excessive great load is applied from a seat occupant's back portion, under inertia of the seat occupant, to both coil springs and spring wires which in turn provide and increase a counter repulsive force against the excessive great load, while being resiliently extended in the rearward direction, so that most of a corresponding great impact to the dorsal and lumbar portions of seat occupant is reduced.
Further, the JP 2006-341802 A teaches a rear-end-collision active linkage provided between a lower area of the support wire element and a headrest movably provided on the top of the seat back, with such an arrangement that, upon occurrence of rear-end collision, the lumbar portion of seat occupant is moved rearwardly to impart a rearward great load through the foam padding and support wire element to a cross portion of the linkage, and at that moment, the linkage is immediately articulated so as to cause concurrent forward movement of the headrest to receive and support a head of the seat occupant.
However, the above-described conventional seat back has been with the problem that, excepting the rear-end collision, for example, during running of vehicle on an uneven road and/or when the vehicle is normally stopped several times on a normal road, both dorsal and lumbar portions of seat occupant are easily swung and rocked on the seat back forwardly and rearwardly, which makes it impossible to keep the seat occupant in a stable and comfortable condition. This unstable seating condition is due to the following facts: i) The afore-said two coil springs, owing to its coil configuration, will be easily extended proportionally responsive to an increased load applied thereto (i.e. responsive to any of low and great loads applied thereto), and therefore, each of the coil springs, upon its receiving a low load several times, is quickly extended and contracted many times, thus causing the seat occupant's dorsal portion to swing and rock in the forward and rearward directions at the upper area of the seat back, and ii) also, each of the afore-said two spring wires, owing to its simple rectilinear configuration, will be easily warped resiliently responsive to a low load applied thereto, as a result of which, such rectilinear spring wire, upon its receiving a low load several times, is immediately extended and contracted numerous times, thus causing the seat occupant's lumbar portion to swing and rock repeatedly in the forward and rearward directions at the lower area of the seat back.
It has been, therefore, desired to provide a seat back of this kind which can prevent or minimize the aforementioned forward-and-rearward swinging or rocking of the seat occupant's dorsal and lumbar portions under a normal condition where a low load is applied to the seat back, and also can absorb the excessive great load caused in the case of rear-end collision or the like.